Commercially available skis are built with an inherent stiffness that is not adjustable. Due to the high cost of skis, the vast majority of skiers can only afford one pair of skis and, consequently, they must decide at the time of purchase how stiff their skis are going to be. Unfortunately, skiing conditions vary widely from ski resort to ski resort, from ski run to ski run, from day to day during the season, and, indeed, from hour to hour during any day. Further, it is almost impossible to purchase skis with the correct stiffness based on the advice of others since a ski stiffness that is suited to one person's skiing style for a given snow condition may not be suited to another's style. All of these factors combine to make it extremely difficult to purchase skis with the right stiffness and as a result, one generally ends up purchasing skis whose stiffness is all right for practically all snow conditions but not exactly right for any particular snow condition and skiing style.
Experienced skiers recognize that skiing conditions at any one resort vary widely over the long periods (e.g., months) of a season so they often own two pairs of skis, a soft pair for powder and a stiffer pair for hard-packed. Such experienced skiers with two or more pairs of skis have the luxury of picking a particular pair to meet a particular day's conditions or because they wish to ski powder or hard-packed that day. However, if the day's conditions change (or if they misjudged the conditions) or if they wish to ski both powder and hard-packed in the same day, then they are stuck with the wrong ski stiffness for the conditions just like the single pair owners.
Skis with adjustable stiffness are contemplated to be an invaluable teaching tool for skiers of all ranges of ability. They are also envisioned as a great aid to skiers whose technique or body characteristics are not perfect. Such skis would be particularly valuable if their stiffness were adjustable lengthwise and crosswise whereby many common problems such as uneven boot cant, uneven strength distribution, and uneven weight distribution could be compensated for in each individual ski. It is quite common for professional skiers, especially ones who have had injuries, to have individual skis custom-built for each leg with each ski having its own stiffness and other performance characteristics. Skis with adjustable stiffness would be ideal for such professionals and, indeed, non-professionals alike.
Several skis with adjustable stiffness have been patented. Notably, U.S. Pat. No. 3,300,226 to Reed, Jr., issued on Jan. 24, 1967, French Pat. No. 1,526,418 to Vogel issued on May 19, 1967, French Pat. No. 1,467,141 to Guey et al issued on Dec. 13, 1965, U.S. Pat. No. 2,918,293 to Tavi issued on Dec. 22, 1959, U.S. Pat. No. 2,258,046 to Clement issued on Oct. 7, 1941, and French Pat. No. 1,118,857 to Michal et al issued on June 12, 1956. Reed and Vogel illustrate the concept of adjusting a ski's stiffness by the use of cylindrical members rotably mounted within bores that extend lengthwise of the ski. Each of the cylindrical members is composed of one or more flat metal, stiffening elements (Reed's member 22 and Vogel's members 2, 4, and 7) mounted in a cylindrical matrix of rubber or epoxy resin. When these flat metal, stiffening elements are disposed vertically, the skis are supposedly stiff and when they are horizontal, the skis are soft. Unfortunately, these two designs have several inherent structural drawbacks. Notably, when the flat metal, stiffening elements are aligned vertically, the ski is more than merely stiff, it is rigid with virtually no flexibility. A flat plate when it is placed edgewise against an applied force as when Reed's and Vogel's metal plates are vertical has virtually no flex. Needless to say, this is totally unacceptable to a skier and goes magnitudes beyond what a skier looks for even in the stiffest of skis. Further, the flexing force of the plates of Reed and Vogel when they are between their vertical and horizontal positions would be at best unpredictable. Finally, a ski flexes thousands of times during a run creating stresses and strains which would tend to tear, crack, and destroy any composite or laminated stiffening element such as Reed's or Vogel's which are composed of metal, rubber, and epoxy all of which have widely varying properties such as expansion and contraction coefficients, strength, flexibility, and the ability to handle stresses and strains. Vogel also teaches the broad idea of varying the stiffness of each of his stiffening elements from one end to the other by progressively diminishing the cross-sectional area of the element. This adds to the range over which he can adjust his stiffness characteristics.
Guey discloses a number of methods for adjusting a ski's stiffness. In the embodiment of his FIGS. 2-7, stiffening rods of circular, T-shaped, and wedge-shaped cross section are mounted lengthwise of the ski. The rods can be made of various materials such as steel, alloys, and plastic and the stiffness is varied by substituting a set of rods of one material for a set of rods of a softer or stiffer material. Guey's method is not much more desirable than owning two different pairs of skis since a skier cannot change the stiffness unless he returns home for a new set of rods or carries a second set with him. Even then, the second set may still be too soft or stiff and a third set needed. Guey's method at best only provides for gross and imprecise adjustments in stiffness. In Guey's embodiments of FIGS. 8-14, the ski has laminated layers which are selectively pulled together to stiffen the ski. Tavi illustrates a manner of adjusting a ski's stiffness by varying the tension on longitudinally extending wires 19. Michal varies his stiffness by adjusting the tension on the rods 2 and Clement varies his between two extremes by either loading or unloading spring 40 as illustrated in his FIGS. 9 and 10.